Legionellae, which consists of the one genus Legionella, are fastidious gram-negative bacteria found in moist environments as intracellular parasites of freshwater protozoa (Fields, et al., 2002, Clin. Microbiol. Rev. 15(3): 506-526). Legionellae can multiply in mammalian cells and cause respiratory disease in humans when a susceptible host inhales or aspirates water or an aerosol containing the bacteria. Although at least 48 species of Legionella are known, L. pneumophila is responsible for most reported cases of legionellosis that result in a severe multisystem disease involving pneumonia, and most other legionellosis cases are caused by L. bozemanii, L. dumoffli, L. longbeachae, and L. micdadei. 
Legionellae may be detected from a number of specimen types and by using a variety of methods. Culture of bacteria from bronchoscopy, bronchoalveolar lavage (BAL), or lung biopsy specimens in a specialized Buffered Charcoal Yeast Extract medium (BCYE) is sensitive and accurate but requires up to two weeks of incubation for maximal recovery followed by identification of the bacteria by using a combination of colony morphology, gram staining, and serologic testing, e.g., immunoassays. Although direct detection of Legionella in uncultured clinical specimens is possible by immunofluorescent or radioimmunoassay methods, these tests are often less sensitive. Legionellosis may be diagnosed by indirect detection of a soluble polysaccharide antigen of L. pneumophila serogroup 1 in urine, but these assays have limited diagnostic utility because of the time delay needed for seroconversion and cannot detect by used for environmental testing. Molecular diagnostic tests have been developed that use DNA probes or a combination of nucleic acid amplification and DNA probes to detect genetic sequences of Legionellae, including the mip gene of L. pneumophila. Such methods detect the presence of nucleic acids from Legionellae in a variety of specimens and with varying degrees of specificity and sensitivity. Many such tests, however, are labor intensive, require at least a day to perform, and are subject to contamination that results in false positive results.
Because Legionellae can survive and persist for a long time in aquatic and moist environments, such as reservoirs and cooling tower water, they can cause community acquired or nosocomial infections. Hence, there is a need for a rapid, sensitive and accurate method to detect Legionellae, particularly L. pneumophila, in environmental samples so that an infectious source can be accurately detected and eliminated to prevent infections. There is also a need for methods that allow rapid and accurate detection of L. pneumophila infections in humans so that infected people may be treated promptly to limit morbidity, mortality, and spread of infection.